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Lipids (/a-level/biology/biological-molecules-and-enzymes/revise-it/lipids)

Carbohydrates contain 3 elements:

1. Carbon (C)2. Hydrogen (H)3. Oxygen (O)

Carbohydrates are found in one of three forms:

1. Monosaccharides2. Disaccharides (both sugars)3. Polysaccharides

Monosaccharides

General formula:.

(CH O)n where n is a number between 3 and 9. They are classi췍ed according to the number ofcarbon atoms. The monosaccharides you will have to know fall into these categories:

Carbohydrates

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carbon atoms. The monosaccharides you will have to know fall into these categories:

C = 3 = triose

C = 4 = tetrose

C = 5 = pentose

C = 6 = hexose

Trioses: (e.g. glyceraldehydes), intermediates in respiration and photosynthesis.

Tetroses: rare.

Pentoses: (e.g. ribose, ribulose), used in the synthesis of nucleic acids (RNA and DNA), co-enzymes (NAD, NADP, FAD) and ATP.

Hexoses: (e.g. glucose, fructose), used as a source of energy in respiration and as buildingblocks for larger molecules.

All but one carbon atom have an alcohol (OH) group attached. The remaining carbon atom hasan aldehyde or ketone group attached.

Chain form:

Ring form:

Due to the bond angles between the carbon atoms, it is possible for pentoses and hexoses toform stable ring structures. The carbon atoms are numbered 1 to 5 in pentoses and 1 to 6 inhexoses.

Depending on the orientation of the OH group on carbon 1, the monosaccharide can have

Depending on the orientation of the OH group on carbon 1, the monosaccharide can haveeither α or β con췍gurations.

Disaccharides and glycosidic bonds

These are formed when two monosaccharides are condensed together. One monosaccharideloses an H atom from carbon atom number 1 and the other loses an OH group from carbon 4to form the bond.

The reaction, which is called a condensation reaction, involves the loss of water (H O) and theformation of an 1,4-glycosidic bond. Depending on the monosaccharides used, this can be anα-1,4-glycosidic bond or a β-1,4-glycosidic bond.

The reverse of this reaction, the formation of two monosaccharides from one disaccharide, iscalled a hydrolysis reaction and requires one water molecule to supply the H and OH to thesugars formed.

Examples of Disaccharides

Sucrose: glucose + fructose,

Lactose: glucose + galactose,

Maltose: glucose + glucose.

Maltose: glucose + glucose.

Sucrose is used in many plants for transporting food reserves, often from the leaves to otherparts of the plant. Lactose is the sugar found in the milk of mammals and maltose is the 췍rstproduct of starch digestion and is further broken down to glucose before absorption in thehuman gut.

Biochemical tests

All monosaccharides and some disaccharides including maltose and lactose are reducingsugars. These can be tested for, by adding Benedict's reagent to the sugar and heating in a

water bath. If a reducing sugar is present, the solution turns green, then yellow and 췍nally

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water bath. If a reducing sugar is present, the solution turns green, then yellow and 췍nallyproduces a brick red precipitate. Non-reducing sugars can also be tested for using Benedict'sreagent but 췍rst require addition of an acid and heating to hydrolyse (break apart) the sugar.The acid must then be neutralised using an alkali like sodium hydroxide before carrying out thetest as described above.

Polysaccharides

Polysaccharide: Function: Structure:Relationship of

structure tofunction:

StarchMain storagepolysaccharidein plants.

Made of 2 polymers - amylose andamylopectin.

Amylose: a polymer of glucoses joinedby α-1,4-glycosidic bonds. Forms ahelix with 6 glucose molecules per turnand about 300 per helix.

Amylopectin: a polymer of glucosesjoined by α-1,4-glycosidic bonds butwith branches of α-1,6-glycosidicbonds. This causes the molecule to bebranched rather than helical

Insoluble thereforegood for storage.

Helix is compact.

The branches meanthat the compoundcan easily hydrolysedto release the glucosemonomers.

Glycogen

Main storagepolysaccharidein animals andfungi

Similar to amylopectin but with manymore branches which are also shorter.

The number andlength of the branchesmeans that it isextremely compactand very fasthydrolysis.

Cellulose

Mainstructuralconstituent ofplant cell walls

Adjacent chains of long, unbranchedpolymers of glucose joined by β-1,4-glycosidic bonds hydrogen bond witheach other to form micro췍brils.

The micro췍brils arestrong and so arestructurally importantin plant cell walls.

Functions of carbohydrates

1. Substrate for respiration (glucose is essential for cardiac tissues).

1. Substrate for respiration (glucose is essential for cardiac tissues).2. Intermediate in respiration (e.g. glyceraldehydes).3. Energy stores (e.g. starch, glycogen).4. Structural (e.g. cellulose, chitin in arthropod exoskeletons and fungal walls).5. Transport (e.g. sucrose is transported in the phloem of a plant).6. Recognition of molecules outside a cell (e.g. attached to proteins or lipids on cell surface

membrane).

Biochemical test

Iodine solution or potassium iodide solution can be used to test for the presence of starch. Apositive result changes the solution from an orange-brown to a blue-black colour. - refer togcse and title biochemical test for carboydrate.

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Lipids (/a-level/biology/biological-molecules-and-enzymes/revise-it/lipids)

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